Heat-curable surface-coating agents, and their use

ABSTRACT

Heat-curable surface-coating agents useful not only for application as solvent-containing finishes and for powder coating but also, in their protonated form, for cathodic electrocoating contain, as a binder, a mixture of (A) a polyadduct, polycondensate or other polymer having an average molecular weight M n  of from 500 to 10,000 and, per molecule, on average two or more OH and/or primary and/or secondary amino groups and (B) a reaction product of (a) one or more polyisocyanates, (b) one or more secondary monoamines and, if appropriate, (c) one or more polyalcohols having two or more hydroxyl groups.

This is a division of application Ser. No. 547,094, filed Oct. 31, 1983,now U.S. Pat. No. 4,576,979, issued on Mar. 18, 1986.

The invention relates to novel heat-curable surface-coating agents whichharden without acid catalysis, namely not only to those heat-curablesurface-coating agents which can be applied conventionally or by meansof powder coating but also, in particular, to those which contain basicgroups, so that they are water-dispersible on protonation with acids andcan be applied by cathodic electrocoating.

German Published Application DAS No. 2,057,799 describes a process forthe cathodic electrophoretic deposition of a water-dispersed ionicorganic resin which comprises a positively charged amine-containingresin and a blocked multifunctional isocyanate. The blocking agentsmentioned in said DAS are aliphatic or cycloaliphatic alcoholscontaining from 2 to 8 carbon atoms, phenol, caprolactam, a C₂ -C₈-aliphatic amine and aliphatic amides. However, the said DAS makes nomention of reacting an isocyanate with a secondary aliphatic amine.While phenol and an aliphatic alcohol are employed as blocking agents inthe examples of this Application, only aliphatic alcohol is used incommercially available products for toxicological reasons. As aconsequence, however, tin salts have to be added as crosslinkingcatalysts and, moreover, hardening has to take place at above 180° C.,which causes yellowing.

It is an object of the present invention to eliminate thesedisadvantages, in particular to lower the baking temperature by aconsiderable amount and hence to cut energy costs.

We have found that this object is achieved by using reaction products of(a) one or more polyisocyanates, (b) one or more secondary monoaminesand, if appropriate, (c) one or more polyalcohols having two or morehydroxyl groups, to prepare heat-curable surface-coating agents whichharden at above 130° C. into hard, resilient films without any hardeningcatalyst having to be added.

Accordingly, the present invention relates to heat-curablesurface-coating agents which contain, as a binder, a mixture of (A) apolyadduct, polycondensate or other polymer having an average molecularweight M_(n) of from 500 to 10,000 and, per molecule, on average two ormore OH and/or primary and/or secondary amino groups and (B) a reactionproduct of (a) one or more polyisocyanates, (b) one or more secondarymonoamines and, if appropriate, (c) one or more polyalcohols having twoor more hydroxyl groups.

The components from which the novel heat-curable surface-coating agentis prepared will now be described in detail:

Component (A) can be a polyadduct, polycondensate or other polymerhaving an average molecular weight M_(n) of from 500 to 10,000 andselected from a very wide range of compound classes. The sole importantpoint is that the polymeric compound has on average two or more OHand/or primary and/or secondary amino groups. Examples of suitablematerials are polyesters, alkyd resins, polyethers, polyacrylate resins,polyurethanes, epoxy resins and their reaction products with alcohols,mercaptans or amines. A further suitable class of compounds arepolydiene resins or oils, for example polybutadiene oils, into which OHgroups can be introduced, for example by adding mercaptoethanol ontosome of the double bonds. Another way of introducing OH groups intopolybutadiene compounds is to react them with maleic anhydride, followedby reacting the product with OH-containing amines, such as ethanolamineor diethanolamine. The required derivatives can also be obtained byepoxidizing polybutadiene oils with per-acids and reacting the productwith amines.

Suitable polyesters have an average molecular weight M_(n) of from 500to 10,000 and a hydroxyl number of from 25 to 400 and are prepared fromaliphatic and/or aromatic dicarboxylic acids of 4 to 10 carbon atoms,e.g. succinic acid, glutaric acid, adipic acid, pimelic acid, subericacid, azelaic acid, sebacic acid, cyclohexanedicarboxylic acid, phthalicacid, isophthalic acid or terephthalic acid, or derivatives thereof, andpolyhydric aliphatic diols, such as ethylene glycol, diethylene glycol,triethylene glycol, polyethylene glycol, propanediol, butanediol,hexanediol, neopentylglycol or neopentylglycol hydroxypivalate and, ifappropriate, more than dihydric alcohols, such as trimethylolpropane,glycerol, erythritol, pentaerythritol, trimethylolbenzene ortrishydroxyethyl isocyanurate.

Suitable alkyd resins have a similar structure, except that they containone or more monocarboxylic acids, for example fatty acids. It is alsopossible to use alkyd resins which contain glycidyl esters of branchedcarboxylic acids.

Examples of suitable polyethers are aliphatic and araliphatic polyetherswhich are obtained by reacting dihydric and/or polyhydric alcohols withvarious amounts of ethylene oxide and/or propylene oxide.

Suitable polyacrylates are OH-containing polyacrylates having a hydroxylnumber of from 25 to 500. They should have an acid number <25,preferably <10, and a Fikentscher K value (3% strength in acetone) offrom 10 to 40, preferably from 12 to 25, and can contain the followingmonomers, for example, as polymerized units:

From 10 to 100% by weight, preferably from 20 to 40% by weight, of oneor more OH- or NH-containing monomers, for exampleisopropylaminopropylmethacrylamide or hydroxy-(C₂ -C₄)-alkyl esters ofan α,β-ethylenically unsaturated carboxylic acid, for example2-hydroxyethyl and hydroxypropyl (meth)acrylates or butanediolmono(meth)acrylate, from 0 to 90% by weight, preferably from 60 to 80%by weight, of one or more ethylenically unsaturated carboxyl- andhydroxyl-free compounds, for example vinylaromatics, such as styrene andvinyltoluene, vinyl esters of carboxylic acids of 2 to 18 carbon atoms,such as vinyl acetate and vinyl propionate, vinyl ethers of monoalkanolsof 1 to 18 carbon atoms, such as vinyl methyl ether and vinyl isobutylether, esters of acrylic acid or methacrylic acid with C₁ -C₁₂-monoalkanols, corresponding diesters of maleic acid, fumaric acid anditaconic acid, (meth)acrylamide, (meth)acrylonitrile, monomers withtertiary amino groups, such as diethylaminoethyl acrylate ordiethylaminoethylamylamide, and mixtures thereof. Another way ofobtaining basic acrylates is to use epoxy-carrying monomers, such asglycidyl methacrylate, and forming an adduct of the oxirane rings of thepolymers with amines.

Examples of suitable polyurethanes are OH-containing polyurethanes whichhave a hydroxyl number of from 25 to 600 and are prepared from aliphaticand/or aromatic diisocyanates, e.g. from tetramethylene diisocyanate,hexamethylene diisocyanate, isophorone diisocyanate, diphenylmethanediisocyanate, toluylene diisocyanate, naphthylene diisocyanate, diphenylether 4,4'-diisocyanate, and any dimers or trimers which can be obtainedtherefrom, and aliphatic diols, such as ethylene glycol, diethyleneglycol, triethylene glycol, polyethylene glycols, propanediol,butanediol, hexanediol, neopentylglycol or neopentylglycolhydroxypivalate and, if appropriate, more than dihydric alcohols, suchas trimethylolpropane, glycerol, pentaerythritol, trimethylolbenzene ortrishydroxyethyl isocyanurate.

Examples of suitable epoxy resins are glycidyl ethers of the typeprepared from 2,2-bis-(4-hydroxyphenyl)-propane and epichlorohydrin.These epoxy resins can have been further modified, for example by havingbeen reacted with polyfunctional alcohols or SH compounds. Examples ofpolyfunctional alcohols suitable for modifying the epoxy resins areethylene glycol, propylene 1,2-glycol, propylene 1,3-glycol andbutane-1,4-diol.

If it is desired to prepare relatively flexible coatings, the epoxyresins can also be modified with longchain polyfunctional alcohols ormercaptans. If the polyfunctional alcohols or mercaptans are used inmore than equivalent amounts, based on the number of epoxy groupspresent, this gives products having terminal OH or SH groups. If,however, they are used in less than equivalent amounts, this givesproducts having terminal epoxy groups which may be reacted further. Themercaptans react with epoxy groups even in the absence of a catalyst,while the reaction of the alcohols requires the use of a catalyst, forexample dimethylbenzylamine, and elevated temperatures, of from about50° to 150° C.

Reaction products of epoxy resins with primary or secondary amines, inparticular with hydroxyl-containing amines, e.g. ethanolamine,methylethanolamine and diethanolamine, can likewise be used as component(A).

If component (A) is a product which contains sufficient amino groups tobe water-soluble or water-dispersible on protonation with acids, theproduct can be combined with component (B) to prepare water-dispersiblebinders for baking finishes, in particular cathodic electrocoatingbinders. The above reaction products of epoxy resins with primary orsecondary amines can be used for this purpose.

Many of the carrier resins proposed for the cathodic electrocoatingprocess can also be used as component (A) in the binders according tothe invention, namely, for example, the reaction products of phenolicMannich bases with epoxy resins as described in German Pat. No.2,419,179, the reaction products of chain-lengthened epoxy resins withsecondary amines as described in U.S. Pat. No. 4,104,147 or reactionproducts of (meth)acrylamidomethylated phenols, amines and epoxy resins,for example as described in German Laid-Open Applications DOS 2,942,488and DOS 3,021,300, as long as they have a molecular weight of from 500to 10,000 and, per molecule, on average two or more OH and/or primaryand/or secondary amino groups. Although the crosslinking activity in thecourse of baking is completely adequate if component (A) contains onlyOH and no primary and/or secondary amino groups, it is frequentlyadvantageous to use products which also contain primary and/or secondaryamino groups, since they can be used to prepare aqueous electrocoatingbaths of high pH, for example pH 6.5-8.0. A high pH, in particular a pHclose to pH 7 or above, helps to prevent corrosion on the electrocoatingequipment. One way of preparing products which have primary andsecondary amino groups and which are suitable for use as component (A)is to react excess primary diamines with epoxy resins and then separateoff the excess amine at elevated temperature and under reduced pressure.

Diamines suitable for the above reaction are especially those which havefrom 2 to 6 carbon atoms, for example ethylenediamine, 1,2- and1,3-diaminopropane, 1,4-diaminobutane and hexamethylenediamine. Thereaction products can, if desired, be chain-lengthened with dicarboxylicacids, for example with sebacic acid or with a fatty acid dimer. Thedesired molecular weight can be set via the ratio of dicarboxylic acidto epoxy resin/amine adduct, for example by employing 1 mole of fattyacid dimer per two molecules of epoxy resin/amine adduct.

A further way of preparing such products with primary amino groups asare suitable for use as component (A) is to react epoxy resins withsecondary amines which contain blocked primary amino groups. Examples ofsuch amines are the diketimine of diethylenetriamine, the ketimine ofaminoethylethanolamine and the ketimine of N-methylethylenediamine. Theketimines can be prepared in a simple manner from the free amines and aketone, for example methyl isobutyl ketone, namely by separating waterout of the refluxing reaction mixture. In the reaction with epoxy resinsonly the secondary amino group reacts, and subsequently the ketimine canbe cleaved by adding water to re-form the free primary amino group.These products too can be flexibilized by reacting some of the primaryamino groups with dicarboxylic acids to lengthen the chain.

Component (A) is generally present in the binder mixture according tothe invention in an amount of from 30 to 95, preferably from 60 to 85%,by weight of the total amount of binder mixture.

Component (B) is a reaction product of (a) one or more polyisocyanates,(b) one or more secondary monoamines and, if appropriate, (c) one ormore polyalcohols having two or more hydroxyl groups.

Any desired aliphatic, alicyclic and/or aromatic polyisocyanate can beused as the isocyanate. Examples of suitable polyisocyanates arediisocyanates, such as hexamethylene diisocyanate, isophoronediisocyanate, cyclohexane 1,4-diisocyanate, toluylene diisocyanate anddiphenylmethane 4,4'-diisocyanate, triisocyanates, such as biuretized orisocyanurated hexamethylene diisocyanate, and the adduct of 3 moles oftoluylene diisocyanate with 1 mole of trimethylolpropane. Evenisocyanate prepolymers, for example adducts of diisocyanates withpolyesterpolyols or polyetherpolyols, can be used as polyisocyanates.

Suitable secondary monoamines are especially secondary aliphatic,cycloaliphatic or araliphatic amines with a boiling point of less than200° C., preferably with a boiling point of from 100° to 200° C.Examples of suitable secondary aliphatic amines are dimethylamine,diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamineand isomers thereof, such as diisopropylamine, including asymmetricalisomers, such as N-ethyl-1-propanamine. Examples of suitablecycloaliphatic and araliphatic amines are dicyclohexylamine andN-methylaniline respectively. Polyalcohols having two or more hydroxylgroups may be used in addition. These polyalcohols can be low molecularweight, defined compounds, such as butane-1,4-diol or hydrogenatedbisphenol A, but it is also possible to use polymeric alcohols, forexample polyesterpolyols or polyetherpolyols having a molecular weightof not more than 10,000.

The subcomponents (a), (b) and (c) are used in such amounts that thetotal number of equivalents of (b) and (c) is not less than equal to theequivalents of (a). If the total number is greater, this gives productswhich still contain free hydroxyl groups; in general, however, the totalnumber of equivalents of (b) and (c) will be chosen to be about equal tothe equivalents of (a).

The reaction is carried out under the conditions customary forisocyanate reactions, namely (a), (b) and (c) can be added in any order,and the reaction temperature can range from room temperature to about150°. If the starting materials and the reaction product are liquid atthe reaction temperature, the reaction can be carried out in the absenceof a solvent, but in general the reaction will be carried out in aninert solvent, such as an ether, ester, ketone or hydrocarbon.

Component (B) is generally used in an amount of from 5 to 70, preferablyfrom 15 to 40%, by weight of the total amount of the binder mixture of(A)+(B).

To prepare the surface-coating agents according to the invention,components (A) and (B) are mixed with each other. If the components areof low viscosity, the mixing can take place in the absence of a solvent,if appropriate by heating the mixture to not more than 130° C. Beforerelatively viscous products are mixed with each other, they aredissolved in conventional organic solvents, e.g. alcohols, ketones,esters, ethers, hydrocarbons etc.

The surface-coating agents according to the invention, in the presenceor absence of pigments, auxiliaries and hardening catalysts, can beapplied to such substrates as wood, plastic or metal by a conventionalmethod, such as spraying, dipping, casting and knife-coating. To preparepowder finishes, components (A) and (B) which are both solid andsolvent-free and whose softening points should in each case be >60° C.,in order to avoid blocking, are dry-milled and mixed with customaryadditives, such as pigments, fillers and leveling agents, for examplethose based on polyacrylate. The mixtures are then homogenized at about100° C. in the melt (extruder). After the extrudates have cooled down toroom temperature they are thoroughly milled. Coarse fractions (above 70μm) are sieved off. The powder finishes are customarily applied byelectrostatic powder spraying.

Products which, by virtue of their amino content, becomewater-dispersible on neutralization with acids, for example acetic acid,can also be employed in the form of an aqueous dispersion. Products ofthis type are advantageously used for electrocoating electricallyconductive substrates, e.g. metal components, sheets or the like ofbrass, copper, or aluminum, metallized plastics or materials which havebeen coated with conductive carbon, and iron and steel which may havebeen chemically pretreated, for example phosphatized. To prepare anelectrocoating bath, an acid, e.g. formic acid, acetic acid or lacticacid, is stirred into the surface-coating agent to neutralize at leastsome of the amino groups, and the mixture is diluted with water to theuse concentration. If the surface coating is to be applied by the EPC(Electro Powder Coating) process in place of the normal cathodicelectrocoating process, component (B) is advantageously used in the formof a powder and component (A) in the form of a water-dispersible carrierresin, and the products used as component (B) have to be solid at roomtemperature.

A cathodic electrocoating bath is generally made up with a solidscontent of from 5 to 30% by weight. The deposition process customarilytakes place at 15°-40° C. in the course of from 1 to 5 minutes at pH4.0-8.5, preferably pH 5.0-7.5, and at deposition voltages of from 50 to500 volts. The electrically conductive body to be coated is connected asthe cathode. The deposited film is hardened at above 130° C. in thecourse of about 20 minutes.

The following examples illustrate the invention without limiting itsscope. Parts and percentages are by weight, unless indicated otherwise.

PREPARATION OF COMPONENT (A) Component A₁

400 parts of hexamethylenediamine are heated to 80° C. An 80% strengthsolution of 400 parts of a commercially available epoxy resin based on2,2-bis-(4-hydroxyphenyl)-propane and having an epoxide equivalentweight of 500 in toluene is then added. Following an initiallyexothermic phase, the reaction is completed at 100° C. in the course of30 minutes. Excess hexamethylenediamine and toluene are then distilledoff under reduced pressure of 15 mbar at an internal temperature as highas 180° C. The remaining traces of free amine are then drawn off at 2.5mbar and 180° C. in a thin-film evaporator. The product has an aminenumber of 169 mg of KOH/g and a softening point of 95° C.

Component A₂

200 parts of component A₁, 30 parts of dimerized fatty acid and 20 partsof xylene are gradually heated to 190° C., water being separated off,and are held at this temperature for one hour. When the mixture hascooled down to 130° C., it is diluted, first with 9 parts of butylglycoland then with 70 parts of isobutanol. The product has a solids contentof 70%. Preparation of component(B)

1 equivalent of the particular polyisocyanate is taken and diluted insufficient isocyanated solvent that the end product has a concentrationof 50%. The secondary amine, with or without the polyol, is added at 80°C. with stirring and the exclusion of moisture in the course of 30minutes. The mixture is then stirred at 80° C. until an NCO value ofless than 1 has been reached.

                  TABLE 1                                                         ______________________________________                                        Compo- 1 equivalent of       Amine  Polyol                                    nent B polyisocyanate                                                                            Solvent   [mol]  [Equivalent]                              ______________________________________                                        B.sub.1                                                                              HDI         Toluene   1 DBA  --                                        B.sub.2                                                                              BHDI        Dioxane   1 DBA  --                                        B.sub.3                                                                              IHDI        Dioxane   1 DBA  --                                        B.sub.4                                                                              TDI         Toluene   1 DHA  --                                        B.sub.5                                                                              TMPTDI      Toluene   1 DBA  --                                        B.sub.6                                                                              IPDI        Dioxane   1 DBA  --                                        B.sub.7                                                                              IHDI        Toluene   0.7 DBA                                                                              0.3 HDL                                   B.sub.8                                                                              TMPTDI      Toluene   0.7 DBA                                                                              0.3 NPDL                                  ______________________________________                                         Abbreviations:                                                                HDI = hexamethylene diisocyanate                                              BHDI = biuretized hexamethylene diisocyanate, 21.9% of NCO                    IHDI = isocyanurated hexamethylene diisocyanate, 22% of NCO                   TDl = toluylene diisocyanate, mixture of 80% of 2,4 and 20% of 2,6isomers     TMPTDI = prepolymer of 1 mol of trimethylolpropane and 3 mol of toluylene     diisocyanate                                                                  IPDI = isophorone diisocyanate                                                DBA = dibutylamine                                                            DHA = dihexylamine                                                            HDL = hexane1,6-diol                                                          NPDL = neopentyl1,3-diol                                                 

EXAMPLE 1

Preparation of a conventionally applied white finish

100 parts of component A₁, 120 parts of ethylglycol, 60 parts ofbutylglycol, 80 parts of titanium dioxide and 100 parts of component B₂are mixed with one another by stirring and heating, and the mixture isthen sand-milled to a DIN 53,203 particle size of <10 μm. The grindingmedium is then separated off to leave a coating solution which has asolids content of 50% and which is knife-coated onto steel sheet in theform of a 100 μm thick wet film which is then baked at 140° C. in thecourse of 20 minutes into a very shiny, hard and resilient paint filmwhich is completely resistant to acetone (100 rubs forwards andbackwards with an acetone-impregnated cottonwool pad).

The following examples demonstrate the use of the heat-curablesurface-coating agents according to the invention in cathodicallydeposited electrocoatings:

EXAMPLE 2

100 parts of component A₂, 60 parts of component B₂ and 2.2 parts ofacetic acid are mixed.

EXAMPLE 3

A binder is prepared by mixing 100 parts of component A₂ with 60 partsof component B₃. The binder is then made water-dilutable by adding 2.2parts of acetic acid.

EXAMPLE 4

100 parts of component A₂ and 60 parts of component B₅ are intimatelymixed. 2.3 parts of acetic acid are then added.

TESTING THE BINDERS

The acid-protonated binders are diluted with water to 10% strengthdispersions to be tested as cationic electrocoatings. 1,000 parts ofeach binder dispersion are admixed, by stirring, with 66 parts of apigment paste which has been prepared as follows: Pigment paste:

A paste binder is prepared as described in Example 1a) of GermanLaid-Open Application DOS 3,121,765, namely by heating 200 parts ofethylene glycol monobutyl ether to 90° C. in a reaction vessel, thenadding dropwise a mixture of 396 parts of N-vinylpyrrolidone, 204 partsof vinyl propionate and 1.2 parts of azobisisobutyronitrile in thecourse of 2 hours, and finally polymerizing at 90° C. for 1 hour. Theresulting solution polymer has a Fikentscher K value of 24. The solidscontent of the copolymer solution is 76%.

250 parts of the above copolymer solution, 210 parts of ethylene glycolmonobutyl ether, 555 parts of ethylene glycol monoethyl ether, 837 partsof water, 1,084 parts of kaolin, 217 parts of basic lead silicate, 145parts of carbon black, 36 parts of rutile and 3,000 parts of glass beadshaving a diameter of 2 mm are stirred at 1,000 r.p.m. for 45 minutes ina stirred ball mill. The glass beads are then separated off to leave ablack paste having a solids content of 50.6%.

The baths are then stirred at 30° C. for 48 hours. Paint films aredeposited at the voltage shown in Table 2 in the course of 2 minutesonto zinc-phosphatized steel test panels connected as the cathode, andare baked at 180° C., at 160° C. and at 140° C. for 20 minutes.Thereafter the baked films are tested for resistance to acetone byrubbing them backwards and forwards 50 times with an acetone-impregnatedcottonwool pad, and the resilience is examined in the form of a reverseimpact test. The following table shows the results.

                                      TABLE 2                                     __________________________________________________________________________    Results of the binder test with a film thickness of 17 μm                                                   Rust bleeding at                                               Acetone resistance                                                                      Reverse                                                                            the crack after                              Coating bath                                                                         Ford throwing power                                                                      at baking tempera-                                                                      impact                                                                             500 hours of ASTM                            of     at voltage ture      in × lb                                                                      salt spray exposure                          __________________________________________________________________________    Example 2                                                                            19.5 cm (250 V)                                                                          160° 1                                                                           160  0.5 mm                                                         140° 1                                                                           160  0.7 mm                                       Example 3                                                                              21 cm (280 V)                                                                          160° 1                                                                           160  0.3 mm                                                         140° 1                                                                           160  0.5 mm                                       Example 4                                                                              22 cm (300 V)                                                                          160° 1                                                                           160  0.4 mm                                                         140° 1                                                                           160  0.5 mm                                       __________________________________________________________________________     Acetone resistance value 1: unassailable                                 

We claim:
 1. A heat-curable surface coating agent which contains, as abinder, a mixture of (A) one or more polyadducts, polycondensates orother polymers having an average molecular weight M_(n) of from 500 to10,000 and, per molecule, on average two or more OH or primary orsecondary amino groups, or two or more OH and primary or secondary aminogroups, or two or more OH and primary and secondary amino groups, withthe proviso that component (A) is at least one compound selected fromthe group consisting of polyester resins including alkyd resinscontaining one or more monocarboxylic acids or glycidyl esters ofbranched carboxylic acids, a polydiene resin and a polydiene oil, and(B) a reaction product of (a) one or more polyisocyanates, (b) one ormore secondary monoamines and (c) one or more polyalcohols having two ormore hydroxyl groups, with the further proviso that component (A) is nota reaction product of epoxy resins with alcohols, mercaptans or amines.2. A surface-coating agent as claimed in claim 1, which contains, ascomponent (A), a polyester resin.
 3. A surface-coating agent as definedin claim 1 whereby (B) is a reaction product of (a) and (b).